Sacrificial layers of selected sacrificial materials are commonly used in fabrications of microstructures, such as microelectromechanical systems and semiconductor devices. One type of sacrificial material is amorphous silicon. Once the desired structures of the microstructure are formed, the sacrificial layers are removed by etching. The success of the etching process depends upon the selectivity of the etching process, wherein the selectivity is defined as a ratio of the amount of sacrificial material being removed to the amount of the structural material being removed. Performance, uniformity and yield can all be improved with increases in the etch selectivity.
More recently, the etching method using selected gas phase etchants has drawn increased interest in fabricating microstructures due to its many advantages, such as potential high selectivity, less contamination and less process stiction as opposed to other possible etching methods, such as a wet etching techniques. In terms of the different ways of feeding the selected gas etchant into the etch chamber containing the microstructure to be etched, the current etching method has two major categories—continuous etchant feeding and one-time (Batch) etchant feeding. In a typical continuous etchant feeding process, the gas etchant flows through the etch chamber until the sacrificial materials of the microstructure are exhausted by the chemical reaction inside the etch chamber. This etch process is unfavorable because of its poor etchant usage efficiency and other disadvantages. In a typical one time etchant feeding process, the selected gas etchant is introduced into the etch chamber at one time and a chemical reaction occurs between the gas etchant and the sacrificial materials inside the etch chamber. This etch feeding technique improves the etchant usage efficiency and the possibility of precise control of the etching process. However, it also has disadvantages. For example, because the gas etchant and the sacrificial materials and the chemical reaction therebetween are confined in the etch chamber throughout the etching process, the etching product (reaction product) will accumulate within the etch chamber. The accumulation may result in the deposition of the etching products on the surface of the microstructure. In addition, because the amount of the etchant fed into the etching system at one time is fixed and the maximum amount of the sacrificial material that can be removed by the fixed amount of the etchant is limited for a given etching system, the maximum amount of the etchant fed into the etching at one time may not be enough to remove a larger amount of the sacrificial material. One approach to solve the etchant quantity limitation is to feed additional amounts of the etchant into the etching system in a discontinuous fashion. For example, in feeding an additional amount of the etchant, the etching system is pumped out and then provided with the additional amount of the etchant. During the pumping out process, the chemical reaction between the etchant and the sacrificial material, thus the etching process, is stopped until the additional amount of the etchant is provided. This feeding process, however, may cause “etch front marks” and/or etching non-uniformities in the microstructures after etch. For example, when the first amount of the etchant fed at one time into the etching system is not enough to remove all sacrificial materials in the microstructure, the boundaries of the sacrificial material (the etch front) may create “marks” in the structures of the microstructure when the chemical reaction (etching process) is stopped due to the lack of the etchant. These “marks” may be permanent throughout and even after the etching process.
In addition to the disadvantages mentioned above, none of these etching processes addresses the issue of non-uniform etch due to the surface variation of the sacrificial material during etching processes. The etch non-uniformity may arise from etch surface variations, especially in an etching process for a wafer having a plurality of dies, each of which has one or more microstructures. In this situation, the dies around the edge of the wafer and the dies near the center of the wafer experience different etching rates. Moreover, the microloading effect can result in etch rate variation within a single die. Since the selectivity often depends upon the etching rate, unstable etching rate may result in etching non-uniformity to the microstructures. As a consequence, the microstructures in different dies may be etched non-uniformly, which is better illustrated in FIG. 1a and FIG. 1b. 
FIG. 1a plots the etching rate and the surface area of the sacrificial materials versus etching time during a typical etching process for a plurality of microstructure dies on a wafer shown in FIG. 1b. During the first 540 seconds, the surface area of the sacrificial materials of all microstructures of the dies deceases slowly with time. Accordingly, the etching rate (plotted in the dotted line) increases to around 22 angstrom/second during the first 160 seconds followed by a slow increase over time thereafter. During the following 160 seconds (from 540 to 700 seconds), the surface area of the sacrificial material presents a steep drop followed by a slow decrease after 700 seconds. Accordingly, the etching rate increases dramatically from 540 to 700 seconds, and increase slowly afterwards. This phenomenon of steep decrease in the surface area and dramatic increase in the etching rate arises from a fact that, during the 240 seconds (from 540 to 780 seconds), the sacrificial material of the dies (plotted in shaded areas in FIG. 1b) around the edge of the wafer is removed due to less sacrificial materials in these dies than those close to the center of the wafer. Because the etch rate is proportional to the concentration of the etchant gas, the rest of the sacrificial materials will experience a much faster etch rate. This etching rate difference may cause unexpected performance differences of the microstructures of the dies around the edge of the wafer and the microstructures of the dies close to the center of the wafer.
Therefore, a method and apparatus is desired for efficiently and uniformly removing sacrificial layers in microstructures using selected gas phase etchant.